Index Table of contents Estimation of Magnetization Direction of 2.5D Bodies from the Black Sea Shelf, Bulgaria, Using Reduced-to-the-Pole and Magnitude Transform Magnetic Anomalies D. Gerovska 1 , M. J. Araúzo-Bravo 2 , P. Stavrev 3 1 Intrepid Geophysics Ltd., North Vancouver, 2 Kyushu Institute of Technology, 3 University of Mining and Geology, Sofia Summary We present a method to estimate the direction of the magnetization vector of 2.5D bodies based upon the correlation between the reduced-to-the-pole field for tentative values of the magnetization direction, and the magnitude transform of the measured magnetic anomaly. We apply the method to find the magnetization vector direction values of the main 2.5D sources causing the magnetic field in the south-east Bulgarian Black Sea shelf. The obtained values are used to model the field sources. Introduction A large part of the observed anomalies from the magnetic map ∆T of the Burgas region and the adjoining Black Sea shelf of Bulgaria cannot be explained with the presence of sources magnetized along the direction of the present magnetic field. This evokes the supposition that the natural remanent magnetization exceeds considerably the induced magnetization. A correct interpretation of the magnetic anomalies requires knowledge about the ratio between the remanent and the induced magnetization, as well as of the direction of the remanent magnetization. In the general case the direction of the remanent magnetization is not parallel to the direction of the present magnetic field. Such remanent magnetization has been measured only for rocks of Quaternary and Late Tertiary age. Pre-Tertiary rocks with normal and reverse magnetization have been discovered, and their magnetization direction differs considerably from that of the modern field (see Blackett, 1956). An example of an anomaly that could not be explained with a source magnetized along the direction of the present magnetic field is the anomaly F (Figure 1, Figure 2a). The pattern of this anomaly suggests that the magnetizatiom direction deviates from the average direction of the geomagnetic field in the region for the epoch of the magnetic survey (D 0 =2.4°, I 0 =59.2°, D 0 and I 0 are the declination and inclination of the inducing geomagnetic field, respectively), since a principal negative anomaly is observed to the south, and a secondary positive anomaly to the north of it. The normal pattern for these latitudes would be a principal positive anomaly. In order to exercise a better control over the estimation of the magnetization direction of the 2.5D anomaly sources in the Black Sea shelf adjoining the Burgas region, a technique was developed to estimate the inclination and declination of the magnetization vector on the bases of the correlation between the reduced-to-the-pole field (RTP) and the magnitude T transform of the anomalous magnetic field. We used the technique to estimate these directions, and employed them in the 2.5D modeling of the sources. Methodology The method we propose is based upon correlating of the magnitude transform T , T = (X 2 +Y 2 +Z 2 ) 1/2 (Stavrev and Gerovska, 2000), and the RTP field for different assumed values of the magnetization directions (D , I, D and I are the declination and inclination of the source magnetization vector, respectively). Both transforms in the case of 2D anomalies would produce positive anomalies, EGM 2007 International Workshop Innovation in EM, Grav and Mag Methods:a new Perspective for Exploration Capri, Italy, April 15 – 18, 2007